Cooling system for piston of internal combustion engine

ABSTRACT

A cooling system for a piston of an internal combustion engine includes a cooling channel ( 34 ) designed as an oil passage embedded in the piston and arranged adjacent to a top ring groove, and an oil supply portion ( 8 ) that supplies oil to the cooling channel. An amount of oil supplied from the oil supply portion to the cooling channel is made larger when an amount of heat generated in a combustion chamber is large than when the amount of heat generated in the combustion chamber is small.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the cooling of a piston in an internalcombustion engine, and more particularly, the cooling of a piston inwhich a cooling channel is formed.

2. Description of Related Art

The piston of an internal combustion engine is fitted with an annularpiston ring having a cut (an abutment). The piston ring may have two endfaces opposite each other via an abutment and provided with elasticresin pieces respectively (e.g., see Japanese Patent ApplicationPublication No. 2010-031789 (JP-A-2010-031789)).

Generally, some of the heat generated during combustion of fuel in thecombustion chamber of a cylinder is transferred to the piston ring viathe piston. Thus, if the amount of the heat generated in the combustionchamber increases, the amount of thermal expansion of the piston ringalso increases. As a result, the width of the abutment (a gap);decreases, and the amount of compression loss and the amount of blow-bygas are reduced.

However, if the amount of the heat generated in the combustion chamberincreases further, the amount of thermal expansion of the piston ringfurther increases. Therefore, the opposite end faces of the abutment maybump against each other. If the opposite end faces of the abutment bumpagainst each other, the stress applied to the piston ring may increaseand thereby cause the contact load between the piston ring and acylinder bore wall surface to increase. These problems are remarkablewhen it comes to the piston ring closest to the combustion chamber (atop ring).

One possible approach to mitigate this problem is to widen the gap ofthe abutment. However, if the amount of the heat generated in thecombustion chamber or when the temperature of the piston is low, theamount of compression loss and the amount of blow-by gas may increasedue to the increased width of the abutment gap.

SUMMARY OF THE INVENTION

The invention reduces the variation width of a temperature of the topring in a system for cooling the piston of an internal combustionengine.

The inventor has found out that the temperature of the top ring can beadjusted with the aid of a cooling channel provided in a piston. Thatis, as a result of Strenuous experiments and verifications, the inventorhas found out that the variation width of the temperature of a pistonring during the operation of the internal combustion engine is reducedby arranging the cooling channel adjacent to a top ring groove of thepiston in which the top ring is fitted and adjusting the amount of oilsupplied to the cooling channel in accordance with the amount of heatgenerated in a combustion chamber.

Thus, according to one aspect of the invention, a cooling system for apiston of an internal combustion engine is equipped with a piston thatincludes a top ring groove provided in an outer peripheral face of thepiston and fitted with a top ring and a cooling channel designed as anoil passage provided in the piston and located adjacent to the top ringgroove, an oil supply portion that supplies oil to the cooling channel,and a control portion that sets an amount of oil supplied from the oilsupply portion to the cooling channel larger when an amount of heatgenerated in a combustion chamber is large than when the amount of heatgenerated in the combustion chamber is small.

The heat generated in the combustion chamber is transferred to a topface of the piston. The heat transferred to the top face of the pistonis transferred in the piston mainly from the top face of the pistontoward the top ring groove, and is discharged from the top ring grooveto a cylinder bore wall surface via the top ring.

When the amount of the heat generated in the combustion chamber islarge, the ratio of the amount of the heat transferred from the top ringto the cylinder bore wall surface to the amount of the heat transferredfrom the piston to the top ring is small. Thus, when the amount of theheat generated in the combustion chamber is large, the amount of rise inthe temperature of the top ring is large.

In contrast, when the amount of the heat generated in the combustionchamber is small, the ratio of the amount of the heat transferred fromthe top ring to the cylinder bore wall surface to the amount of the heattransferred from the piston to the top ring is large. Thus, when theamount of the heat generated in the combustion chamber is small, theamount of rise in the temperature of the top ring is small.

As described hitherto, the temperature of the top ring greatly changesin accordance with the amount of the heat generated in the combustionchamber. When the temperature of the top ring greatly changes, the sizeof the abutment gap also greatly changes correspondingly. Thus, in thecase where the top ring is formed such that the abutment gap assumes asuitable size when the temperature of the top ring is low, opposed endfaces of the abutment bump against each other when the temperature ofthe top ring becomes high. In contrast, when the top ring is formed suchthat the abutment gap assumes a suitable size when the temperature ofthe top ring is high, the abutment gap becomes excessively wide when thetemperature of the top ring becomes low.

In contrast, in the case where the cooling channel is arranged adjacentto the top ring groove, especially in the case where the cooling channelis arranged on a transfer path from the top face of the piston to thetop ring groove, the heat transferred from the top face of the piston tothe top ring groove is absorbed by the oil in the cooling channel.

Accordingly, in the case where the amount of the oil flowing through thecooling channel is made large when the amount of the heat generated inthe combustion chamber is large, the amount of that heat traveling fromthe top face of the piston toward the top ring groove which is absorbedby the oil in the cooling channel becomes large. Thus, the amount of theheat transferred from the top face of the piston to the top ring groovecan be prevented from becoming excessively large. As a result, thetemperature of the top ring can be prevented from becoming excessivelyhigh when the amount of the heat generated in the combustion chamber islarge.

However, when the amount of the oil flowing through the cooling channelis made small when the amount of the heat generated in the combustionchamber is small, the amount of that heat traveling from the top face ofthe piston toward the top ring groove which is absorbed by the oil inthe cooling channel becomes small. Thus, the amount of the heattransferred from the top face of the piston to the top ring groove canbe prevented from becoming excessively small. As a result, thetemperature of the top ring can be prevented from becoming excessivelylow when the amount of the heat generated in the combustion chamber issmall.

It should be noted that most of the heat transferred from the combustionchamber to the piston may be discharged to the cylinder bore wallsurface when the amount of the heat generated in the combustion chamberis extremely small (e.g., when the internal combustion engine isoperated at low load and low rotational speed). In such a case, thetemperatures of the piston and the top ring may fall after temporarilyrising.

In this view, the control portion according to the aspect of theinvention may set the amount of the oil supplied from the oil supplyportion to the cooling channel to zero (stop the oil supply portion)when the amount of the heat generated in the combustion chamber is equalto or smaller than a predetermined lower limit. “The lower limit”mentioned herein is a value at which the temperature of the top ring isconsidered to become lower than a presupposed temperature range (atemperature range where the abutment gap of the top ring assumes asupposed size), and is determined in advance through an adaptationprocessing with the aid of an experiment or the like.

When the oil supply portion is stopped, the amount of the heatdischarged from the piston to oil is substantially zero. Further, whenthe oil supply portion is stopped, the interior of the cooling channelis filled with air. The air in the cooling channel functions as a heatinsulation layer for reducing or shutting off the heat transferred fromthe top face of the piston to the top ring groove. Thus, the amount ofthe heat discharged from the piston to the cylinder bore wall surfacedecreases.

As described hitherto, when the amount of the heat discharged from thepiston to oil and the amount of the heat discharged from the piston tothe cylinder bore wall surface are reduced, the temperature of thepiston (particularly a region around the top ring groove) is restrainedfrom falling. When the temperature of the region around the top ring isrestrained from falling, the temperature of the top ring is alsorestrained from falling correspondingly.

As described above, when the amount of the heat supplied from the oilsupply portion to the cooling channel is adjusted, the temperature ofthe top ring is held equal to a substantially constant temperature(hereinafter referred to as “a suitable temperature”). As a result,during the operation of the internal combustion engine, the size of theabutment gap can be held substantially constant.

When the size of the abutment gap of the top ring is held substantiallyconstant during the operation of the internal combustion engine, the topring can be designed such that the abutment gap assumes a desired sizeat the aforementioned suitable temperature. As a result, the amount ofcompression loss and the amount of blow-by gas can also be minimizedregardless of the amount of the heat generated in the combustionchamber.

It should be noted herein that the amount of the heat generated in thecombustion chamber is correlated with the amount of fuel injection.Thus, the control portion may adjust the amount of the oil supplied fromthe oil supply portion to the cooling channel using the amount of fuelinjection as a parameter. Further, since the amount of fuel injection isdetermined using an engine load and an engine speed as parameters, thecontrol portion may adjust the amount of the oil supplied from the oilsupply portion to the cooling channel using the engine speed and theengine load as parameters.

In the meantime, the pressure in a space surrounded by the top ring, thepiston, and the cylinder bore (hereinafter referred to as “a firstspace”) changes substantially in synchronization with changes in thepressure in the combustion chamber. In contrast, the pressure in a spacesurrounded by the top ring, the second ring, the piston, and thecylinder bore (hereinafter referred to as “a second space”) changes witha delay from changes in the pressure in the combustion chamber. The timelag in this case increases as the flow rate of the blow-by gas flowingfrom the abutment gap of the top ring into the second space decreases.That is, the aforementioned time lag increases as the abutment gap ofthe top ring decreases.

Accordingly, when the abutment gap of the top ring is made as narrow aspossible, the aforementioned time lag becomes long. Thus, the pressurein the second space may become higher than the pressure in the firstspace. In such a case, since the top ring floats up in the top ringgroove, the sealability of the top ring may also deteriorate.

It should be noted that the phenomenon in which the pressure in thesecond space becomes higher than the pressure in the first space islikely to be caused when the amount of the heat generated in thecombustion chamber is large. This is considered to result from the factthat the outer diameter of a second land of the piston (a region betweenthe top ring groove and the second ring groove) increases to reduce thevolume of the second space when the amount of the heat generated in thecombustion chamber is large.

Thus, the cooling channel according to the aspect of the invention maybe so formed as to be located adjacent to the second land as well as thetop ring groove. According to this construction, when the amount of theheat generated in the combustion chamber is large, the heat of thesecond land is absorbed by the oil in the cooling channel. As a result,the temperature of the second land is restrained from rising.

When the temperature of the second land is restrained from rising, theouter diameter of the second land is restrained from increasing (thesecond land is restrained from thermally expanding). As a result, thedecrease in the volume of the second space is alleviated. When thedecrease in the volume of the second space is alleviated, the phenomenonin which the pressure in the second space becomes higher than thepressure in the first space is unlikely to be caused.

Further, the control portion according to the aspect of the inventionmay control the oil supply portion such that the amount of the oilsupplied from the oil supply portion to the cooling channel becomeslarger when the internal combustion engine is being warmed up than whenthe internal combustion engine includes been warmed up, for anequivalent engine load and an equivalent engine speed.

The temperature difference between the piston and the cylinder bore islarge when the internal combustion engine is being warmed up. This isbecause the piston is directly warmed by the heat generated in thecombustion chamber but the cylinder bore is indirectly warmed receivingthe heat discharged from the piston. Furthermore, since the cylinderbore is larger in thermal capacity than the piston, the speed of rise inthe temperature of the cylinder bore is lower than the speed of rise inthe temperature of the piston.

When the temperature difference between the piston and the cylinder boreis large, the piston is expanded (the outer diameter of the piston isincreased) whereas the cylinder bore is hardly expanded (the innerdiameter of the cylinder bore is hardly increased). Thus, the clearancebetween the piston and the cylinder bore and the clearance between thepiston ring and the cylinder bore are small. As a result, the piston,the piston ring, the cylinder bore, and the like may be abraded, and thedegree of friction therebetween may be increased.

In this view, in the case where the amount of the oil supplied from theoil supply portion to the cooling channel is made larger when theinternal combustion engine is being warmed up than when the internalcombustion engine includes been warmed up, the thermal expansion of thepiston is alleviated. As a result, the aforementioned problem can beprevented from being caused.

Further, the control portion according to the aspect of the inventionmay continue to operate the oil supply portion when the temperature ofcoolant for the internal combustion engine is equal to or higher than apredetermined upper-limit coolant temperature or when the temperature ofoil (the oil temperature) is equal to or higher than a predeterminedupper-limit oil temperature. In other words, the oil supply portion maybe prohibited from stopping when the coolant temperature is equal to orhigher than the upper-limit coolant temperature or when the oiltemperature is equal to or higher than the upper-limit oil temperature.“The upper-limit coolant temperature” mentioned herein and “theupper-limit oil temperature” mentioned herein are obtained bysubtracting a predetermined margin from a temperature at which theinternal combustion engine is considered to be overheated and atemperature at which an oil film is considered to be brokenrespectively. When the oil supply portion is thus controlled, theinternal combustion engine can be prevented from being overheated, andthe oil film can be prevented from being broken.

According to the above aspect of the invention, in the system forcooling the piston of the internal combustion engine, the variationwidth of the temperature of the top ring can be reduced. Thus, theabutment gap of the top ring can be held equal to a preferableclearance, and the amount of compression loss and the amount of blow-bygas can be made as small as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 shows the overall structure of a cooling system for a pistonaccording to the first embodiment of the invention;

FIG. 2 is a cross-sectional view of the piston according to the firstembodiment of the invention;

FIG. 3 shows the correlation between a temperature difference ΔT, anengine load Q, and an engine speed Ne;

FIG. 4 is a view schematically showing a map prescribing a relationshipamong the amount of oil injection, the engine load Q, and the enginespeed Ne in the first embodiment of the invention;

FIG. 5 is a schematic view of the heat transfer path in the piston;

FIG. 6 is a cross-sectional view of a piston according to the secondembodiment of the invention;

FIG. 7 is an enlarged view of the gap between the piston and a cylinderbore wall surface;

FIG. 8 shows changes in the pressure Pv1 in a first space and changes inthe pressure Pv2 in a second space;

FIG. 9 is a schematic view of a phenomenon in which a top ring floatsup;

FIG. 10 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Nein the third embodiment of the invention;

FIG. 11 is shows the changes in a coolant temperature over time;

FIG. 12 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Nein when an internal combustion engine is steadily operated while beingwarmed up;

FIG. 13 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Newhen the internal combustion engine is operated under intermediate loadand at intermediate rotational speed while being warmed up;

FIG. 14 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Newhen the internal combustion engine is operated under high load and athigh rotational speed while being warmed up;

FIG. 15 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Newhen the coolant temperature is lower than that in the example shown inFIG. 12;

FIG. 16 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Newhen the coolant temperature is lower than that in the example shown inFIG. 13; and

FIG. 17 is a schematic view of a map prescribing a relationship amongthe amount of oil injection, the engine load Q, and the engine speed Newhen the coolant temperature is lower than that in the example shown inFIG. 14.

DETAILED DESCRIPTION OF EMBODIMENTS

Example embodiments of the invention will be described below withreference to the drawings. The dimensions, materials, shapes, relativearrangement and the like of components described in the exampleembodiments of the invention are not intended to limit the technicalscope of the invention thereto unless otherwise specified.

The first embodiment of the invention will be described on the basis ofFIGS. 1 to 5. FIG. 1 shows the overall structure of an internalcombustion engine to which the invention is applied. FIG. 2 is across-sectional view of a piston according to the first embodiment ofthe invention.

An internal combustion engine 1 may be a compression-ignition internalcombustion engine (a diesel engine) having a plurality of cylinders 2.It should be noted that only one of the plurality of the cylinders 2 isshown in FIG. 1. A piston 3 is slidably fitted in each cylinder 2 of theinternal combustion engine 1 so that the piston may reciprocate in theaxial direction of the cylinder. The piston 3 is coupled to a crankshaft(not shown) via a connecting rod 4.

A generally cylindrically combustion chamber 30 is formed in the topface of the piston 3. In addition, three annular grooves 31, 32, and 33are formed in an outer peripheral face of the piston 3. The annulargroove 31 is located closest to a top dead center (at the highestposition in FIG. 2), and is fitted with a top ring 5 (the annular groove31 will be referred to hereinafter as the “top ring groove 31”). Theannular, groove 32 is located directly below the top ring groove 31 andit fitted with a second ring 6 (the annular groove 32 will be referredto hereinafter as the “second ring groove 32”). The annular groove 33 islocated closest to a bottom dead center (at the lowest position in FIG.2), and is fitted with an oil ring 7 (hereinafter, the annular groove 33will be referred to as the “oil ring groove 33”). It should be notedthat the top ring 5, the second ring 6, and the oil ring 7 are annularmembers equipped with abutments.

The top ring groove 31 is formed in the outer peripheral face of anabrasion-resistant loop 300 that is cast in the piston 3. Theabrasion-resistant loop 300 is an annular member formed of a materialharder and more resistant to abrasion than the piston 3 (e.g., aNi—Cr—Cu cast iron material).

A hollow abrasion-resistant loop 310 is cast in an inside of theabrasion-resistant loop 300. The hollow abrasion-resistant loop 310 isan annular member that is U-shaped in cross-section and has an openingportion on an outer periphery side thereof. The outer periphery side ofthe hollow abrasion-resistant loop 310 abuts on an inner peripheral faceof the abrasion-resistant loop 300. That is, the opening portion of thehollow abrasion-resistant loop 310, which is U-shaped in cross-section,is closed up by the inner peripheral face of the abrasion-resistant loop300. An annular space 34 surrounded by the hollow abrasion-resistantloop 310 and the abrasion-resistant loop 300 functions as a passage foroil supplied from a later-described oil jet 8 (the space 34 will bereferred to hereinafter as “a cooling channel 34”).

Communication passages 35 and 36 through which an opening portion formedthrough a bottom face of the piston 3 communicates with the coolingchannel 34 are formed through the piston 3. The communication passage 35as one of the communication passages 35 and 36 functions as a passagethrough which the oil injected from the oil jet 8 is introduced into thecooling channel 34 (the communication passage 35 will be referred tohereinafter as “an introduction passage 35”). The communication passage36 as the other of the communication passages 35 and 36 functions as adischarge passage from which the oil flowing out from the coolingchannel 34 is discharged (the communication passage 36 will be referredto hereinafter as “a discharge passage 36”).

The internal combustion engine 1 is equipped with the oil jet 8, whichinjects oil from a bottom dead center side to a top dead center side inthe cylinder 2. It should be noted that the oil jet 8 is so arranged asto be located below the piston 3 when the piston 3 is located at thebottom dead center. Furthermore, the oil jet 8 is arranged and formedsuch that the oil injected from the oil jet 8 is oriented toward theintroduction passage 35.

The oil jet 8 communicates with an oil pan 10 via a supply passage 9.The supply passage 9 is provided at a midway position thereof with anoil pump 11 that sucks up the oil in the oil pan 10. A flow rateadjusting valve 12 is arranged in the supply passage 9 between the oiljet 8 and the oil pump 11. The flow rate adjusting valve 12 is a valvethat adjusts the amount of the oil flowing in the supply passage 9. Theamount of the oil injected from the oil jet 8 (the amount of oilinjection) is increased or reduced through the adjustment of the flowrate of the oil in the supply passage 9 by the flow rate adjusting valve12. It should be noted that the oil jet 8 functions as the oil supplyportion of the invention.

It should be noted that an electrically operated valve mechanism whoseratio between an open-valve time and a closed-valve time is subjected toduty control or an electrically operated valve mechanism whose openingdegree can be changed continuously or stepwise can be employed as theflow rate adjusting valve 12. Further, the flow rate adjusting valve 12may be a valve mechanism including a check valve that opens when thepressure of oil is equal to or higher than a certain value and apressure adjusting valve that adjusts the pressure of the oil in thesupply passage 9.

The supply passage 9 is provided with a return passage 13 that bypassesthe oil pump 11. This return passage 13 is a passage for returning asurplus amount of oil from that region of the supply passage 9 which islocated downstream of the oil pump 11 to that region of the supplypassage 9 which is located upstream of the oil pump 11. A one-way valve(a check valve) 14 that allows only the flow of oil from that region ofthe supply passage 9 which is located downstream of the oil pump 11toward that region of the supply passage 9 which is located upstream ofthe oil pump 11 is arranged in the return passage 13.

The internal combustion engine 1 thus constructed is accompanied by anECU 15. The ECU 15 is an electronic control unit equipped with a CPU, aROM, a RAM, a backup RAM, and the like. Output signals of varioussensors sucincludes a coolant temperature sensor 16, a crank positionsensor 18, an accelerator position sensor 19, an oil temperature sensor20, and the like are input to the ECU 15.

The coolant temperature sensor 16 is a sensor that outputs an electricsignal correlated with a temperature of the coolant circulating throughthe internal combustion engine 1. The crank position sensor 18 is asensor that outputs an electric signal correlated with a rotationalposition of a crankshaft. The accelerator position sensor 19 is a sensorthat outputs an electric signal correlated with a depression amount ofan accelerator pedal (an engine load). The oil temperature sensor 20 isa sensor that outputs an electric signal correlated with a temperatureof the oil circulating through the internal combustion engine 1 (an oiltemperature).

On the basis of the output signals of the aforementioned varioussensors, the ECU 15 performs the control of the amount of the oilsupplied from the oil jet 8 to the cooling channel 34 (which will bereferred to hereinafter as “oil jet control) as well as known types ofcontrol such as fuel injection control and the like. A method ofperforming oil jet control will be described hereinafter. It should benoted that the control portion according to the invention is realizedthrough the performance of oil jet control by the ECU 15.

Oil jet control according to this embodiment of the invention isdesigned to adjust the amount of oil injection from the oil jet 8 suchthat the temperature of the top ring 5 becomes substantially constant.That is, oil jet control according to this embodiment of the inventionis designed to adjust the amount of oil injection from the oil jet 8such that the abutment gap of the top ring 5 becomes substantiallyconstant.

The temperature of the top ring 5 changes in accordance with the amountof the heat generated in the combustion chamber 30. For example, whenthe amount of the heat generated in the combustion chamber 30 is large,the amount of the rise in the temperature of the piston 3 is large.Therefore, the amount of the rise in the temperature of the top ring 5is also large correspondingly.

When the temperature of the top ring 5 becomes high, the top ring 5thermally expands to narrow the abutment gap. When the temperature ofthe top ring 5 further rises, opposed end faces of the abutment gap bumpagainst each other to generate a force acting to increase the outerdiameter of the top ring 5.

In this case, when the temperature of an inner wall surface of thecylinder 2 (a cylinder bore wall surface) is high, the aforementionedforce is counterbalanced due to an increase in the inner diameter of thecylinder 2. However, when the temperature of the cylinder bore wallsurface is lower than the temperature of the piston 3 and the differencebetween the temperatures is large, the top ring 5 and the cylinder 2 aretightened. Thus, the stress applied to the top ring 5 may becomeexcessively large, or the contact load between the top ring 5 and thecylinder bore wall surface may become excessively large.

Thus, the size of the abutment gap needs to be determined such that theopposed end faces of the abutment do not press each other when thetemperature of the top ring 5 is high and the temperature of thecylinder bore wall surface is low. However, in the case where the sizeof the abutment gap is determined according to this method, when thetemperature of the top ring 5 is low, the abutment gap may becomeexcessively wide to cause a compression loss or an increase in theamount of blow-by gas.

Thus, in the oil jet control according to this embodiment of theinvention, the ECU 15 so adjusts the amount of oil injection as tosuppress the rise in the temperature of the top ring 5 when the amountof the heat generated in the combustion chamber 30 is large (when thedifference in temperature between the piston 3 and the cylinder borewall surface is large) and suppress the fall in the temperature of thetop ring 5 or promote the rise in the temperature of the top ring 5 whenthe amount of the heat generated in the combustion chamber 30 is small(when the difference in temperature between the piston 3 and thecylinder bore wall surface is small).

The amount of the heat generated in the combustion chamber 30 changes inaccordance with the amount of the fuel burned in the combustion chamber30, namely, the amount of fuel injection. In principle, the amount offuel injection is determined using a load Q of the internal combustionengine 1 (an engine load) and a rotational speed Ne of the internalcombustion engine 1 (an engine speed) as parameters. Thus, in thisembodiment of the invention, an example in which the amount of oilinjection is adjusted using the engine load Q and the engine speed Ne asparameters will be described.

FIG. 3 is a view showing a relationship among a temperature differenceΔT, the engine load Q, and the engine speed Ne. “The temperaturedifference ΔT” mentioned herein is a difference between the temperatureof the piston 3 (preferably the temperature of the top ring groove 31)and the temperature of the cylinder bore wail surface.

In FIG. 3, when the engine load Q and the engine speed Ne are low, theamount of the heat generated in the combustion chamber 30 is smallerthan when the engine load Q and the engine speed Ne are high. Thus, thetemperature difference ΔT is small, and the abutment gap of the top ring5 is wide.

In contrast, if the engine load Q and the engine speed Ne are both high,the amount of the heat generated in the combustion chamber 30 is largerthan when the engine load Q and the engine speed Ne are low. Thus, thetemperature difference ΔT is large, and the abutment gap of the top ring5 is narrow.

In this view, the ECU 15 controls the flow rate adjusting valve 12 suchthat the amount of oil injection from the oil jet 8 becomes larger whenthe amount of the heat generated in the combustion chamber 30 is largethan when the amount of the heat generated in the combustion chamber 30is small. In other words, the ECU 15 controls the flow rate adjustingvalve 12 such that the amount of oil injection from the oil jet 8becomes larger when the temperature difference AT is large than when thetemperature difference ΔT is small.

More specifically, the ECU 15 may control the flow rate adjusting valve12 according to a map shown in FIG. 4. The map shown in FIG. 4 is a mapdetermining a relationship among the engine load Q, the engine speed Ne,and the amount of oil injection.

In FIG. 4, when the engine load Q and the engine speed Ne are high (aregion A in FIG. 4), the amount of oil injection is set to a maximamount. When the engine load Q and the engine speed Ne are low (a regionC in FIG. 4), the amount of oil injection is set equal to zero (the oiljet 8 is stopped). However, oil may be injected for the purpose oflubricating a space between the piston 3 and the cylinder bore wallsurface or lubricating a space between the piston 3 and the connectingrod 4. Further, when the engine load Q and the engine speed Ne are in aregion between the region A and the region C (a region B in FIG. 4), theamount of oil injection is made smaller than the aforementioned maximumamount. It should be noted that the region C in FIG. 4 is a region wherethe amount of the heat generated in the combustion chamber 30 is equalto or smaller than a lower limit “The lower limit” mentioned herein is avalue at which the temperature of the top ring 5 may be lower than alater-described suitable temperature.

It should be noted herein that part of the heat generated in thecombustion chamber 30 is transferred from the top face of the piston 3toward the top ring groove 31 and discharged from the top ring groove 31to the cylinder bore wall surface, More specifically, as shown in FIG.5, the heat transferred from inside the combustion chamber 30 to thepiston 3 is mainly transferred from an upper edge portion 30 a of thecombustion chamber 30 in the piston 3 toward the top ring groove 31 (theabrasion-resistant loop 300) (see an arrow in FIG. 5). In this view,when the cooling channel 34 is arranged inside the top ring groove 31,this cooling channel 34 is located on a path of the heat. In otherwords, it is preferable that the cooling channel 34 be arranged on thepath of the aforementioned heat.

Thus, in the case where the amount of oil injection is set to themaximum amount when the engine load Q and the engine speed Ne are high,most of the heat traveling from the upper edge portion 30 a toward thetop ring groove 31 is absorbed by the oil in the cooling channel 34. Asa result, the temperatures of the piston 3 and the top ring groove 31are restrained from rising, and the temperature of the top ring 5 isalso restrained from rising correspondingly.

In the case where the temperature of the top ring 5 is restrained fromrising when the engine load Q and the engine speed Ne are high, theopposed end faces of the abutment of the top ring 5 can be preventedfrom bumping against each other. Therefore, the contact load between thetop ring 5 and the cylinder bore wall surface can be prevented frombecoming excessively large.

However, if the amount of oil injection is set to zero when the engineload Q and the engine speed Ne are low, the interior of the coolingchannel 34 is filled with air. The air in the cooling channel 34functions as a heat insulating layer that shuts off the heat travelingfrom the upper edge portion 30 a toward the top ring groove 31. Thus,the amount of the heat discharged from the piston 3 to the cylinder borewall surface decreases. As a result, the temperatures of the piston 3and the top ring groove 31 are restrained from falling, and thetemperature of the top ring 5 is also restrained from fallingcorrespondingly.

In the case where the temperature of the top ring 5 is restrained fromfalling when the engine load Q and the engine speed Ne are low, theabutment gap of the top ring 5 can be prevented from becomingexcessively wide. Thus, an increase in the amount of compression lossand an increase in the amount of blow-by gas can be avoided. Further,when the temperature of the top ring groove 31 is restrained fromfalling, the temperature of the atmosphere in a gap (a crevice) betweena top land of the piston 3 and the cylinder bore wall surface is heldhigh. When the temperature of the atmosphere in the crevice is high, thetemperature of the gas flowing through the abutment gap of the top ring5 is also higher than when the temperature of the atmosphere in thecrevice is low. As a result, the mass of the gas flowing through theabutment gap of the top ring 5 further decreases.

In the case where the amount of oil injection is made smaller than themaximum amount when the engine load Q and the engine speed Ne are in anintermediate load/intermediate rotational speed range, the amount of theheat absorbed by oil from the piston 3 is prevented from becoming muchlarger than the amount of the heat traveling from the upper edge portion30 a toward the top ring groove 31. As a result, the piston 3 and thetop ring groove 31 are restrained from being overcooled, and the topring 5 is also restrained from being overcooled. It should be noted thatthe amount of oil injection in the aforementioned region B of FIG. 4 maybe a fixed amount, but may also be an amount that is changed inaccordance with the engine load Q and the engine speed Ne. The amount ofoil injection in this case may be made larger when the engine load Q ishigh than when the engine load Q is low, and may be made larger when theengine speed Ne is high than when the engine speed Ne is low. Further,the amount of oil injection is increased as an amount of fuel injectionincreases.

The temperature of the top ring 5 can be held substantially constant (atthe suitable temperature) regardless of the operation state of theinternal combustion engine 1, through the performance of oil jet controlby the ECU 15 as described above. “The suitable temperature” mentionedherein is a temperature at which the abutment gap is the narrowestwithin such a range that the opposed end faces of the abutment of thetop ring 5 do not bump against each other. It should be noted that thetop ring 5 is designed such that the abutment gap has a desired size atthe aforementioned suitable temperature.

In consequence, the cooling system for the piston according to thisembodiment of the invention makes it possible to prevent the abutmentgap of the top ring 5 from becoming excessively narrow to cause theopposed end faces of the abutment to bump against each other when theamount of the heat generated in the combustion chamber 30 is large, andto prevent the abutment gap of the top ring 5 from becoming excessivelywide to cause an increase in the amount of compression loss or anincrease in the amount of blow-by gas when the amount of the heatgenerated in the combustion chamber 30 is small.

It should be noted that although the example in which the oil jet 8 isstopped when the engine load Q and the engine speed Ne are low has beendescribed in this embodiment of the invention, the oil jet 8 may beprohibited from being stopped when an output signal of the coolanttemperature sensor 16 (a coolant temperature) indicates a temperatureequal to or higher than an upper-limit coolant temperature or when anoutput signal of the oil temperature sensor 20 (an oil temperature)indicates a temperature equal to or higher than an upper-limit oiltemperature. “The upper-limit coolant temperature” mentioned herein and“the upper-limit oil temperature” mentioned herein are obtained bysubtracting a predetermined margin from a temperature at which theinternal combustion engine may be overheated or a temperature at whichan oil film may be broken respectively. When the oil jet 8 is thusprohibited from being stopped, the internal combustion engine 1 can beprevented from being overheated, and the oil film can be prevented frombeing broken.

Next, the second embodiment of the invention will be described on thebasis of FIGS. 6 to 9. Only the structural details of the secondembodiment that differ from those of the first embodiment of theinvention will be described below.

The difference between the first embodiment of the invention and thesecond embodiment of the invention lies in the structure of the coolingchannel. In the first embodiment of the invention, the cooling channelis arranged to cool the top ring groove in a concentrated manner.However, in the second embodiment, the cooling channel is arranged tocool a second land 37 as well as the top ring groove.

FIG. 6 is a cross-sectional view of the piston 3 according to the secondembodiment of the invention. In FIG. 6, components identical to those ofthe first embodiment (see FIG. 2) are denoted by the same referencesymbols. The abrasion-resistant loop 300 according to this embodimentextends further in the axial direction of the cylinder than that of thefirst embodiment. More specifically, the abrasion-resistant loop 300 hasa width ranging from the top land of the piston 3 to a third landthereof.

A second ring groove 32, as well as the top ring groove 31, is formed inthe abrasion-resistant loop 300. Accordingly, the abrasion-resistantloop 300 located between the top ring groove 31 and the second ringgroove 32 serves also as the second land 37.

The hollow abrasion-resistant loop 310, which is substantially equal inwidth to the abrasion-resistant loop 300 of the piston 3, is cast in theinside of the abrasion-resistant loop 300. The hollow abrasion-resistantloop 310 is an annular member with a U-shaped cross-section as in thefirst embodiment of the invention. The opening portion of the hollowabrasion-resistant loop 310 is closed up by the inner peripheral face ofthe abrasion-resistant loop 300. The annular space 34 surrounded by thehollow abrasion-resistant loop 310 and the abrasion-resistant loop 300functions as a cooling channel.

In the case where the same oil jet control as in the first embodiment ofthe invention is performed for the piston 3 thus constructed, when theamount of the heat generated in the combustion chamber 30 is large (whenthe engine load Q and the engine speed Ne are high), the temperature ofthe top ring 5 is restrained from rising, and the temperature of thesecond land 37 is also restrained from rising.

It should be noted herein that FIG. 7 is an enlarged view of a gapbetween the piston 3 and a cylinder bore wall surface. In FIG. 7, V1denotes a space (a first space) surrounded by the top ring 5, the piston3 (the top land), and the cylinder bore wall surface. In FIG. 7, V2denotes a space (a second space) surrounded by the top ring 5, thepiston 3 (the second land 37), the second ring 6, and the cylinder borewall surface.

As shown in FIG. 8, a pressure Pv1 in the first space V1 changessubstantially in synchronization with the pressure in the combustionchamber 30 (see a solid line in FIG. 8). In contrast, a pressure Pv2 inthe second space V2 changes with a delay from the pressure in thecombustion chamber 30 (see the single-dash chain line in FIG. 8). Thetime lag in this case increases as the abutment gap of the top ring 5decreases. Thus, as described in the first embodiment of the invention,when the abutment gap of the top ring 5 is made as narrow as possible,there may be a period in which the pressure Pv2 in the second space V2is higher than the pressure Pv1 in the first space V1 (see a hatchedregion in FIG. 8).

Especially when the engine load Q and the engine speed Ne are high, inother words, when the difference in temperature between the second land37 and the cylinder bore wall surface is large, the volume of the secondspace V2 is reduced, so that the pressure Pv2 in the second space V2 islikely to become higher than the pressure Pv1 in the first space V1.

When the pressure Pv2 in the second space V2 becomes higher than thepressure Pv1 in the first space V1, a phenomenon in which the top ring 5floats up to the top dead center side in the direction of the axis ofthe cylinder in the top ring groove 31 occurs as shown in FIG. 9. Whenthe top ring 5 thus floats up, blow-by gas may leak from the gap betweenthe top ring 5 and the top ring groove 31.

In contrast, in the case where the temperature of the second land 37 isrestrained from rising when the engine load Q and the engine speed Neare high, the outer diameter of the second land 37 is restrained fromincreasing, or the outer diameter of the second land 37 is reduced. Inthis case, the volume of the second space V2 is restrained fromdecreasing, or the volume of the second space V2 is increased. As aresult, the pressure Pv2 in the second space V2 is unlikely to rise.

Further, when the temperature of the second land 37 is restrained fromrising, the volume of the gas present in the second space V2 is alsorestrained from increasing. As a result, the pressure Pv2 in the secondspace V2 is more unlikely to rise. Furthermore, the second ring groove32 and the second ring 6 are cooled in no small measure by the oil inthe cooling channel 34. Therefore, the abutment gap of the second ring 6increases. When the abutment gap of the second ring 6 increases, the gasin the second space V2 is discharged from the abutment gap. As a result,the pressure Pv2 in the second space V2 can be more reliably preventedfrom becoming higher than the pressure Pv1 in the first space V1.

According to the embodiment of the invention described above, thepressure Pv2 in the second space V2 is prevented from becoming higherthan the pressure Pv1 in the first space V1. Therefore, in addition toan effect equivalent to that of the first embodiment of the invention,the phenomenon in which the top ring 5 floats up can also be suppressed.As a result, the amount of blow-by gas can be restrained from increasingdue to a deterioration in the sealability of the top ring 5.

Next, the third embodiment of the invention will be described on thebasis of FIG. 10. In this case, the constructional details differentfrom those of the first embodiment of the invention will be described,and the constructional details identical to those of the firstembodiment of the invention will not be described.

The difference between the first embodiment of the invention and thisembodiment of the invention consists in that the method of performingoil jet control is changed depending on whether the internal combustionengine 1 is being warmed up or has been warmed up. The piston 3 issmaller in thermal capacity than the cylinder block. Furthermore, whilethe cylinder bore wall surface indirectly receives the heat of thecombustion chamber 30 via the piston 3 and a piston ring, the piston 3directly receives the heat of the combustion chamber 30. Thus, thetemperature difference ΔT between the piston 3 and the cylinder borewall surface is likely to be larger when the internal combustion engine1 is being warmed up than when the internal combustion engine 1 has beenwarmed up.

When the temperature difference ΔT between the piston 3 and the cylinderbore wall surface becomes large, the difference between the amount ofthe increase in the outer diameter of the top ring 5 and the amount ofthe increase in the inner diameter of the cylinder 2 increases. As aresult, the stress applied to the top ring 5 may become excessivelylarge, or the contact load between the top ring 5 and the cylinder borewall surface may become excessively large.

The ECU 15 controls the oil jet 8 so that the oil injection amount asthe internal combustion engine 1 is being warmed up is greater than thatwhen the internal combustion engine 1 has been warmed up. FIG. 10 is aschematic view of a map shows the correlation between the oil injectionamount, the engine load Q, and the engine speed Ne. In FIG. 10, theborders between the regions A, B, and C when the internal combustionengine 1 is being warmed up are shown with solid lines. In FIG. 10, theborders between the regions A, B, and C when the internal combustionengine 1 has been warmed up are shown with single-dash chain lines.

As shown in FIG. 10, the respective borders shift toward a lowload/speed side when the internal combustion engine 1 is being warmed upthan when the internal combustion engine 1 has been warmed up.Therefore, the oil injection amount from the oil jet 8 is greater whenthe internal combustion engine 1 is being warmed up than when theinternal combustion engine 1 has been warmed up.

As a result, when the internal combustion engine 1 is being warmed up,the difference between the amount of the increase in the outer diameterof the top ring 5 and the amount of the increase in the inner diameterof the cylinder 2 is prevented from becoming larger than when theinternal combustion engine 1 has been warmed up. Thus, even when theinternal combustion engine 1 is being warmed up, the application ofexcessive stress to the top ring 5 may be prevented, and the contactload between the top ring 5 and the cylinder bore wall surface can beprevented from becoming excessively large, while minimizing theincreases in compression loss and the amount of blow-by gas.

Next, the fourth embodiment of the invention will be described on thebasis of FIGS. 11 to 17. In following description, only the structuraldetails the fourth embodiment that differ from those of the thirdembodiment of will be described.

In the third embodiment of the invention, the method of performing oiljet control when the internal combustion engine 1 is steadily operatedwhile being warmed up has been described. However, in the fourthembodiment of the invention, a method of performing oil jet control whenthe internal combustion engine 1 is transiently operated during warm upwill be described.

When the internal combustion engine 1 is transiently operated after coldstart, there occurs a situation in which the temperature of the cylinderbore wall surface (the cylinder block) hardly rises although thetemperature of the piston 3 rapidly rises. In such a case, thetemperature difference ΔT between the piston 3 and the cylinder borewall surface may become larger. When the temperature difference ΔTbetween the piston 3 and the cylinder bore wall surface becomes large,the difference between the amount of the increase in the outer diameterof the top ring 5 and the amount of the increase in the inner diameterof the cylinder 2 increases. As a result, the stress applied to the topring 5 may become excessively large, or the contact load between the topring 5 and the cylinder bore wall surface may become excessively large.This problem becomes more remarkable as the rate of the rise in thetemperature of the piston 3 (the speed at which the temperature rises)increases and as the temperature of the cylinder bore wall surfacefalls.

Thus, in this embodiment of the invention, the method of performing oiljet control is changed using the rate of the rise in the temperature ofthe piston 3 and the temperature of the cylinder bore wall surface asparameters. For example, the ECU 15 controls the flow rate adjustingvalve 12 such that the amount of oil injection becomes larger when therate of the rise in the temperature of the piston 3 is high and thetemperature of the cylinder bore wall surface is low than when the rateof the rise in the temperature of the piston 3 is low and thetemperature of the cylinder bore wall surface is high. The engine load Qand the engine speed Ne remain equivalent regardless of whether “therate of the rise in the temperature of the piston 3 is high and thetemperature of the cylinder bore wall surface is low” as mentionedherein or “the rate of the rise in the temperature of the piston 3 islow and the temperature of the cylinder bore wall surface is high” asmentioned herein.

The rate of the rise in the temperature of the piston 3 is correlatedwith the rate of the rise in the coolant temperature. Thus, the amountof change in the coolant temperature per a certain time can be used asthe rate of the rise in the temperature of the piston 3. Further, thetemperature of the cylinder bore wall surface is substantially equal tothe temperature of the coolant flowing through the cylinder block. Thus,the output signal of the coolant temperature sensor 16 (the coolanttemperature) can be used as the temperature of the cylinder bore wallsurface.

FIG. 11 shows the changes in the coolant temperature over time. Thetwo-dash chain line X1 in FIG. 11 shows changes in the coolanttemperature when the internal combustion engine 1 is steadily operatedwhile being warmed up. The single-dash chain line X2 in FIG. 11indicates changes in the coolant temperature when the internalcombustion engine 1 is operated at intermediate load/speed while beingwarmed up. In FIG. 11, changes in the coolant temperature when theinternal combustion engine 1 is operated at high load/speed while beingwarmed up are shown with a solid line X3. In addition, thw0 shows thecoolant temperature during the performance of oil jet control. In FIGS.11, ΔP1, ΔP2, and ΔP3 indicate the amounts of change in the coolanttemperature (the rates of increase in the temperature) for apredetermined time t for each of X1, X2, and X3.

As indicated by the two-dash chain line X1 in FIG. 11, when the internalcombustion engine 1 is steadily operated while being warmed up, the ECU15 controls the amount of oil injection in accordance with the map shownin FIG. 12. It should be noted that the map shown in FIG. 12 isequivalent to the map described in the third embodiment of the invention(see FIG. 10), and that the borders A, B, and C are shifted toward thelow load/speed side relative to when the internal combustion engine 1has been warmed up.

Then, the rate ΔP2 of temperature increase when the internal combustionengine 1 is operated at intermediate load/speed while being warmed up ishigher than the rate ΔP1 temperature increase when the internalcombustion engine 1 is idling while being warmed up. Thus, when theinternal combustion engine 1 is operated at intermediate load/speedwhile being warmed up, the temperature difference ΔT between the piston3 and the cylinder block is greater than that when the internalcombustion engine 1 is idling while being warmed up.

Thus, as indicated by the single-dash chain line X2 in FIG. 11, when theinternal combustion engine 1 is operated at intermediate load/speedwhile being warmed up, the ECU 15 controls the amount of oil injectionin accordance with the map shown in FIG. 13. The solid lines in FIG. 13indicate borders between the regions A, B, and C when the internalcombustion engine 1 is operated at intermediate load/speed while beingwarmed up. The chain lines in FIG. 13 indicate the borders between theregions A, B, and C when the internal combustion engine 1 is steadilyoperated while being warmed up.

In FIG. 13, the borders between the regions A, B, and C when theinternal combustion engine 1 is operated at intermediate while beingwarmed up is shifted toward the low load/speed side relative to theborders between the regions A, B, and C when the internal combustionengine 1 is steadily operated while being warmed up. Thus, more oil isinjected when the internal combustion engine 1 is operated atintermediate load/speed during warm-up than when the internal combustionengine 1 is steadily operated during warm-up.

Further, the rate ΔP3 at which temperature increases when the internalcombustion engine 1 is operated at high load/speed during warm up ishigher than the rate ΔP2 at which temperature increases when theinternal combustion engine 1 is operated at intermediate load/speedduring warm up. Thus, if the internal combustion engine 1 is operated athigh load/speed during warm up, the temperature difference ΔT betweenthe piston 3 and the cylinder block is expected to become larger thanwhen the internal combustion engine 1 is operated at intermediateload/speed during warm up.

Thus, as indicated by the solid line X3 in FIG. 11, when the internalcombustion engine 1 is operated at high load/speed during warm up, theECU 15 controls the amount of oil injected in accordance with to a mapshown in FIG. 14. The solid line in FIG. 14 indicates The border betweenthe regions A and B when the internal combustion engine 1 is operated athigh load/speed during warm up. The chain line in FIG. 14 indicates theborder between the regions A and B when the internal combustion engine 1is operated at intermediate load/speed while being warmed up.

In FIG. 14, the border between the regions A and B when the internalcombustion engine 1 is operated at high load/speed during warm up isshifted toward the low load/speed side relative to the border betweenthe regions A and B when the internal combustion engine 1 is operated atintermediate load/speed while being warmed up. Furthermore, in the mapshown in FIG. 14, a region for stopping the oil jet 8 (a regioncorresponding to the region C in each of FIGS. 12 and 13) is eliminated.That is, even when the internal combustion engine 1 makes a transitionfrom a high load/high rotational speed operation range to a low load/lowrotational speed operation range, a small amount of oil is injected fromthe oil jet 8.

Thus, the amount of oil injection in the case where the internalcombustion engine 1 is operated at high load/high rotational speed whilebeing warmed up is larger than the amount of oil injection in the casewhere the internal combustion engine 1 is operated at intermediateload/intermediate rotational speed while being warmed up. As a result,the temperature difference ΔT between the piston 3 and the cylinder borewall surface can be restrained from increasing.

It should be noted that the temperature difference ΔT between the piston3 and the cylinder bore wall surface may become larger when the coolanttemperature (the temperature of the cylinder bore wall surface) duringthe performance of oil jet control is lower than the aforementionedvalue thw0 than when the coolant temperature is equal to thw0. It isthus desirable that the amount of oil injection be made larger when thecoolant temperature is lower than thw0 than when the coolant temperatureis equal to thw0.

For example, when the internal combustion engine 1 is steadily operatedwhile being warmed up, the ECU 15 controls the amount of oil injectionaccording to a map shown in FIG. 15. It should be noted that solid linesin FIG. 15 indicate the borders between the regions A, B, and C in thecase where the coolant temperature is lower than thw0 respectively, andthat single-dash chain lines in FIG. 15 indicate the borders between theregions A, B, and C in the case where the coolant temperature is equalto thw0 respectively (which are equivalent to the borders between theregions A, B, and C in FIG. 12 respectively).

In FIG. 15, the borders between the regions A, B, and C in the casewhere the coolant temperature is lower than thw0 shift more to the lowload/low rotational speed side than the borders between the regions A,B, and C in the case where the coolant temperature is equal to thw0respectively. Thus, the amount of oil injection in the case where theinternal combustion engine 1 is steadily operated while being warmed upincreases as the coolant temperature falls. As a result, even when thecoolant temperature (the temperature of the cylinder bore wall surface)becomes low, the temperature difference ΔT between the piston 3 and thecylinder bore wall surface is restrained from increasing.

Further, when the internal combustion engine 1 is operated atintermediate load/intermediate rotational speed while being warmed up,the ECU 15 controls the amount of oil injection according to a map shownin FIG. 16. It should be noted that a solid line in FIG. 16 indicatesthe border between the regions A and B in the case where the coolanttemperature is lower than thw0, and that the single-dash chain line inFIG. 16 indicate the border between the regions A and B in the casewhere the coolant temperature is equal to thw0 (which is equivalent tothe border between the regions A and B in FIG. 13).

In FIG. 16, the border between the regions A and B in the case where thecoolant temperature is lower than thw0 shifts more toward the lowload/low rotational speed side than the border between the regions A andBin the case where the coolant temperature is equal to thw0.Furthermore, in the map shown in FIG. 16, a region for stopping the oiljet 8 (a region corresponding to the region C in FIG. 13) is eliminated.Thus, the amount of oil injection in the case where the internalcombustion engine 1 is operated at intermediate load/intermediaterotational speed while being warmed up increases as the coolanttemperature falls. As a result, even when the coolant temperature (thetemperature of the cylinder bore wall surface) becomes low, thetemperature difference ΔT between the piston 3 and the cylinder borewall surface is restrained from increasing.

Furthermore, when the internal combustion engine 1 is operated at highload/high rotational speed while being warmed up, the ECU 15 controlsthe amount of oil injection according to a map shown in FIG. 17. In themap shown in FIG. 17, a region in which a small amount of oil isinjected from the oil jet 8 (a region corresponding to the region B inFIG. 14) is eliminated. That is, the amount of oil injection is setequal to a maximum amount in all operation ranges of the internalcombustion engine 1. Thus, even in the case where the internalcombustion engine 1 is operated at high load/high rotational speed whenthe internal combustion engine 1 is being warmed up and the temperatureof the cylinder bore wall surface is low, the temperature difference ΔTbetween the piston 3 and the cylinder bore wall surface can berestrained from increasing.

According to the embodiment of the invention described above, even inthe case where the rate ΔP of the rise in the temperature of the piston3 becomes high when the internal combustion engine 1 is being warmed up,the temperature difference ΔT between the piston 3 and the cylinder borewall surface can be restrained from increasing. As a result, the amountof the increase in the outer diameter of the top ring 5 can be heldsmall. Thus, the stress applied to the top ring 5 can be prevented frombecoming excessively large, and the contact load between the top ring 5and the cylinder bore wall surface can be prevented from becomingexcessively large.

It should be noted that although the example in which a changeover amongthe maps is made using the coolant temperature during the performance ofoil jet control and the rate ΔP of the rise in the coolant temperatureas parameters has been described in this embodiment, of the invention, afunction expression covering the aforementioned relationships shown inFIGS. 12 to 17 may be used. That is, the amount of oil injection may bedetermined using a function expression whose arguments are the coolanttemperature, the rate of the rise in the temperature, the engine load Q,and the engine speed Ne.

Further, in this embodiment of the invention, the example in which therate of the rise in the coolant temperature is used as the rate of therise in the temperature of the piston 3 has been described. However,some time lag may be produced until changes in the temperature of thepiston 3 are reflected by the coolant temperature.

Thus, it is also appropriate to calculate an amount of the heattransferred from the combustion chamber 30 to the piston 3 using theamount of fuel injection as a parameter, and make a changeover among themaps using a result of the calculation and the temperature of thecylinder bore wall surface (the coolant temperature) as parameters. Inthis case, an amount Hq of the heat transferred from the combustionchamber 30 to the piston 3 per a certain time tinj may be calculated onthe basis of an expression shown below.

Hq=Hinj×j∫(ΣFinj)dt+tinj

In the aforementioned expression, Hinj represents a small amount (J/g)of heat generation of fuel, and ΣFinj represents a sum of amounts Finjof fuel injection within the certain time tinj.

The ECU 15 may control the flow rate adjusting valve 12 such that theamount of oil injection becomes larger when the amount Hq of the heatcalculated according to the aforementioned expression is large and thecoolant temperature (the temperature of the cylinder bore wall surface)is low than when the amount Hq of the heat is small and the coolanttemperature (the temperature of the cylinder bore wall surface) is high.

According to this method, oil jet control can be performed in accordancewith the actual temperature of the piston 3.

It should be noted that the ECU 15 may simultaneously calculate anamount of oil injection on the basis of the rate ΔP of the rise in thecoolant temperature and calculate an amount of oil injection on thebasis of the amount Hq of the heat transferred from the combustionchamber 30 to the piston 3, and may control the flow rate adjustingvalve 12 in accordance with the larger one of two results of thecalculation. According to this method, the temperature difference ΔTbetween the piston 3 and the cylinder bore wall surface can be morereliably restrained from increasing.

Next, the fifth embodiment of the invention will be described. In thiscase, the constructional details different from those of the firstembodiment of the invention will be described, and the constructionaldetails identical to those of the first embodiment of the invention willnot be described.

In this embodiment of the invention, an example in which oil jet controlis performed when the cylinder bore wall surface is overcooled as in acase where there is a malfunction in the cooling system of the internalcombustion engine 1, especially a case where a thermostat valve seizesin an open-valve state will be described.

In the case where the thermostat valve seizes in an open-valve state,even when the coolant temperature is lower than a valve-openingtemperature (or a valve-closing temperature) of the thermostat valve,the coolant flows through a radiator. Thus, the coolant temperature mayfurther fall. When the coolant temperature thus falls, the cylinder borewall surface is overcooled.

In the case where the cylinder bore wall surface is overcooled, evenwhen the amount of the heat generated in the combustion chamber 30 issmall, the temperature difference ΔT between the piston 3 and thecylinder bore wall surface may increase. When the temperature differenceΔT between the piston 3 and the cylinder bore wall surface increases,the amount of increase in the outer diameter of the top ring 5 maybecome excessively large with respect to the amount of increase in theinner diameter of the cylinder 2. As a result, even when the amount ofthe heat generated in the combustion chamber 30 is small, the stressapplied to the top ring 5 may become excessively large, or the contactload between the top ring 5 and the cylinder bore wall surface maybecome excessively large.

In this view, during oil jet control according to this embodiment of theinvention, the ECU 15 sets the amount of oil injection to the maximumamount regardless of the operation state (the engine load Q and theengine speed Ne) of the internal combustion engine 1 as in the case ofthe aforementioned map of FIG. 17 when there is a malfunction in thecooling system.

In this case, as a method of detecting a malfunction in the coolingsystem, it is possible to use a method in which it is determined thatthere is a malfunction in the cooling system when the rate of fall inthe coolant temperature (the speed at which the temperature falls) ishigher than a predetermined upper-limit rate of fall or when the amountof fall in the coolant temperature is larger than a predeterminedupper-limit amount of fall. “The upper-limit rate of fall” mentionedherein is a rate of fall in the case where there is a malfunction in thethermostat valve in the open-valve state or a value obtained bysubtracting a predetermined margin from this rate of fall. Further, “theupper-limit amount of fall” may be an amount of fall in temperature inthe case where the thermostat valve seizes in the open-valve state or avalue obtained by subtracting a predetermined margin from this amount offall in temperature.

According to this embodiment of the invention, when there is amalfunction in the cooling system, the temperature difference betweenthe piston 3 and the cylinder bore wall surface can be prevented fromincreasing. As a result, the outer diameter of the top ring 5 isrestrained from increasing. Thus, when the amount of the heat generatedin the combustion chamber 30 is small, the stress applied to the topring 5 can be prevented from becoming excessively large, and the contactload between the top ring 5 and the cylinder bore wall surface can beprevented from becoming excessively large.

It should be noted that at least two or all of the first to fifthembodiments of the invention can be combined with one another. As aresult, the abutment gap of the top ring 5 can be held substantiallyconstant in various cases, for example, a case where the internalcombustion engine 1 is being warmed up, a case where the internalcombustion engine 1 has been warmed up, a case where there is amalfunction in the cooling system of the internal combustion engine 1,and the like.

Further, in each of the first to fifth embodiments of the invention, thecooling channel 34 is constructed of the hollow abrasion-resistant loop.However, any construction may be adopted as long as the cooling channel34 is arranged adjacent to the top ring 5 (and the second land 37).

1. A cooling system for a piston of an internal combustion engine,comprising: a piston that includes a top ring groove provided in anouter peripheral face of the piston and fitted with a top ring, and acooling channel designed as an oil passage embedded in the piston andlocated adjacent to the top ring groove; an oil supply portion thatsupplies oil to the cooling channel; and a control portion thatincreases an amount of oil supplied from the oil supply portion to thecooling channel as an amount of heat generated in a combustion chamberincreases.
 2. The cooling system for the piston of the internalcombustion engine according to claim 1, wherein the control portionstops supplying oil from the oil supply portion to the cooling channelwhen the amount of heat generated in the combustion chamber is equal toor smaller than a predetermined lower limit.
 3. The cooling system forthe piston of the internal combustion engine according to claim 1,wherein the control portion increases the amount of oil supplied fromthe oil supply portion to the cooling channel as an amount of fuelinjection increases.
 4. The cooling system for the piston of theinternal combustion engine according to claim 1, wherein the controlportion increases the amount of oil supplied from the oil supply portionto the cooling channel as an engine speed and an engine load increase.5. The cooling system for the piston of the internal combustion engineaccording to any one of claims 1 to 4, wherein the cooling channel is soformed as to be located adjacent to the top ring groove and a secondland.
 6. The cooling system for the piston of the internal combustionengine according to claim 5, wherein the piston is further equipped witha second ring groove located directly below the top ring groove, and thesecond land is provided between the top ring groove and the second ringgroove.
 7. The cooling system for the piston of the internal combustionengine according to any one of claims 1 to 6, wherein the controlportion makes the amount of oil supplied from the oil supply portion tothe cooling channel larger when the internal combustion engine is beingwarmed up than when the internal combustion engine includes been warmedup, for an equivalent engine load and an equivalent engine speed.
 8. Thecooling system for the piston of the internal combustion engineaccording to any one of claims 1 to 7, wherein the control portionprohibits the oil supply portion from being stopped when a temperatureof coolant is equal to or higher than a predetermined upper-limitcoolant temperature or when a temperature of oil is equal to or higherthan a predetermined upper-limit oil temperature.
 9. The cooling systemfor the piston of the internal combustion engine according to any one ofclaims 1 to 8, wherein the oil supply portion is an oil jet.
 10. Thecooling system for the piston of the internal combustion engineaccording to any one of claims 1 to 9, wherein the top ring groove andthe cooling channel are annular.
 11. A method of controlling a coolingsystem for a piston of an internal combustion engine, comprising:providing a piston that includes a top ring groove provided in an outerperipheral face of the piston and fitted with a top ring, and a coolingchannel designed as an oil passage embedded in the piston and locatedadjacent to the top ring groove, and an oil supply portion that suppliesoil to the cooling channel; and making an amount of oil supplied fromthe oil supply portion to the cooling channel larger when an amount ofheat generated in a combustion chamber is large than when the amount ofheat generated in the combustion chamber is small.
 12. The method ofcontrolling the cooling system for the piston of the internal combustionengine according to claim 11, wherein the amount of oil supplied fromthe oil supply portion to the cooling channel is made larger when theinternal combustion engine is being warmed up than when the internalcombustion engine includes been warmed up, for an equivalent engine loadand an equivalent engine speed.